Method of making moulds



METHOD OF MAKING MOULDS Noel Shaw, Ivy Lea, Haydon Bridge, England,assignor, by mesne assignments, to Shaw Process Development Corp., PortWashington, N. Y.

No Drawing. Application April 9, 1953, Serial No. 347,829

6 Claims. (Cl. 22-192) This invention broadly relates to the process ofwhat is known as precision casting of metals and metal alloys, andparticularly pertains to the method of making moulds and to mouldseffected by that method and which are especially adapted for theproduction of high precision castings from ferrous and non-ferrousmetals or materials which cannot be conveniently die cast, as disclosedin copending British patent application numbered 8,556, dated April 3,1952, for Improvements in or Relating to Moulds for Metal Casting.

Precision casting, with the exclusion of die casting, is a relativelynew art, and therefore it seems proper to evaluate the same for thepurpose of facilitating a comparison between the prior art and thepresent invention.

Of the known methods presently used the following are the mostnoteworthy:

, (a) Investment casting, the so-called lost wax method, wherein a waxpattern is placed within a refractory mould, whereupon the wax is meltedand the metal to be cast is poured into the mould;

(b) The mercury pattern process where frozen mercury is employed as apattern similar to the lost wax method;

The shell method of casting which comprises the production of shell-likemoulds of sand bonded with a thermo-setting resin; and

(d) The method of providing refractory moulds by covering a pattern witha refractory paste-like mixture and permitting that mixture to set,whereupon the pattern is removed by opening the mould and the mouldmaterial is then subjected to a slow drying and a careful heating orbaking process before being used for pouring the metal into the mould.

The lost wax method is applicable for casting of relatively small volumeand weight. The mercury process allows making castings of larger sizes,and the shell mould process allows the casting of still larger pieces.

The method of making refractory moulds and slowly drying and heating orbaking the mould material is most suitable for making fairly largecastings but possesses definite drawbacks and disadvantages. Since thatprocess approaches closest the present disclosure, the same shall bemore fully described. I

It provides the making of refractory moulds for casting metals by mixingcomminuted refractory material into a paste or slurry with an organicsilicate and sufiicient water to hydrolyse the silicate, and to whichmixture is added a gelling accelerator which may be of acid or basicorigin such as hydrochloric acid, lime or magnesia, respectively. Fromthe resulting semi-liquid paste or slurry the mould is formed byintimately applying the paste to the pattern, whereupon the mouldmaterial is allowed to set, such setting being effected by the inducedhydrolysis and the gelling of the silicate. When thus set the mould isstripped or opened and the pattern is removed. The opened mould partsare now subjected to a lengthy drying-out process and to a carefulheating procedure with the employment of gradually rising temperaturesin order United States Patent C 7 2,795,022 Patented June 11, 1957 iceto avoid thermal shocks of any kind which could cause the mould materialto crack.

This procedure has been followed since it was generally assumed that theremoval of volatiles, such as alcohol formed by the hydrolysis, and thewater vapor would cause disruption or distoration of the set mouldunless such removal of volatiles and water vapor is carried outcarefully in a slow drying and careful baking procedure.

The aforesaid cautious treatment of the said mould requires considerablespace for the storage of moulds during drying and consumes a relativelylong time, and in addition considerable furnace space is lost by thelengthy period of slow baking or firing.

In spite of the aforesaid precautions it has been found that mouldsproduced by that method often crack or become bodily distorted due toshrinkage, thereby causing undesirable dimensional changes in the mould.

From the foregoing it becomes very clear that the afore-describedheretofore used method of producing moulds is not reliable and thereforeis not applicable for continuous and precise foundry production.

In the preparation of the mould paste such as described above, variousorganic silicates have been employed, but on account of its cheapnessand availability ethyl orthosilicate has been mostly used. In thepresent invention it is contemplated to employ ethyl orthosilicate,although it will become obvious that any other alkyl silicate whichyields an alcohol on hydrolysis and which alcohol is sufficientlyvolatile to burn when ignited, can be readily substituted; any of suchusable silicates shall be hereinafter termed a low alkyl silicate.

As refractory material for the type of moulds indicated there have beenused a variety of refractory factors such as silica, alumina, zirconin,zirconia, and like refractory oxides and silicates, the choice of therefractory material being determined by the casting temperature of themetal to be cast.

Contrary to the heretofore used procedure of slow drying and carefulfiring of set moulds, the principal object of the present inventioninvolves the method of rapidly removing volatiles from the mould partsafter the organic silicate binder has set by hydrolysis and gelling.

Another object of this invention is the provision of making mouldmaterial by the preferred employment of acid gelling accelerators, asagainst basic accelerators.

A still further object of this invention is the provision of arelatively simple and inexpensive method of making peculiarly crazed ormicro-cracked moulds of high porosity, which latter however will notallow cast metal to fill the crack formation but which facilitates theready escape of gases during pouring.

The foregoing and numerous other objects and additional advantages ofthe present invention will become more fully understood from thefollowing description.

The method of the present invention is based upon the new finding thatif the casting mould is stripped after it sets by gelling and thereafteris immediately fired while free air circulation about all surfaces ofthe mould is provided, the disadvantages encountered with theaforedescribed known method of slow drying and careful baking the mouldare completely obviated.

The general procedure in accordance with the present method consists ofproducing a refractory mould for metal casting by mixing comminutedrefractory material with a suitable binder, by adding thereto a desired,heretofore defined lower alkyl silicate and an aqueous acid or gellingaccelerator, all of which ingredients are mixed to a semi-fluidhomogenous paste consistency. Now the pattern for the desired casting iscovered by the paste to produce a mould, which latter is allowed to setby gelling. When set, the mould is stripped or opened and the patternis. removed and the set mould portions are now placed upon a supportingsurface which will permit free air circulation about all surfaces of themould parts, whereupon the alcohol or any other burnable volatilesformed by the hydrolysis of the binder are ignited as they escape fromall surfaces of the mould. It is to be noted that such ignition of thevolatiles takes place immediately after the mould is set by gelling andthat the ignition and burning of the volatiles occurs evenly at allparts of the mould surfaces due to the free air circulation about them.Such ignition or burning of the volatiles is preferably effected by thesimultaneous application of flames to all surfaces of the mould,including inner portions thereof, so that all of the escaping volatilescontinue to burn by themselves.

When burning of the volatiles ceases, the mould is assembled and themetal is poured at once, although in some cases, particularly where thecasting includes thin sections requiring the metal to be kept in itsmolten state for a relatively long time period, or when the castingmetals have a high melting point, it is desirable to submit the mould tofurther heating in a suitable furnace, such as of the muffie type, inwhich case the mould, after firing, can be placed directly into thefurnace at its full temperature for the casting.

In describing the present method more specifically, the refractorymaterial used is formed into a paste or slurry with, for example, ethylsilicate and an aqueous acid, such as hydrochloric acid, by pouring theingredients into suitable containers, as is well known in the art (seefor example Prange, U. S. Patent No. 1,909,008). After the formation ofthe mould about the pattern, the mould material sets by gelling of thebinder and the mould is stripped, the mould parts are at once placedupon a suitable grid to provide air circulation about all surfaces ofthe mould parts, whereupon the mould parts are rapidly fired in that thevolatiles are ignited by the simultaneous application of intense flamesto all surfaces of the mould parts. The burning of the formed andescaping volatiles is allowed to continue preferably under a hood toremove the fumes, and when the burning is seen to have ceased, the mouldparts can be at once assembled and the metal at once poured. When themetal is congealed, the mould material is removed by cracking it loosefrom the cast.

Due to the rapid firing of the mould by flash-burning of the volatiles,as above described, a mould structure of a specific character isobtained which is highly advantageous to successful casting. The rapidloss of the volatiles from the set mould material effects a peculiartype of internal shrinkage characterized by an internal micro-crackingof the mould material which occurs without causing the slightestexternal dimensional changes of the. mould. Thus the mould materialattains a state of minute, uniform porosity. Due to the internalmicrocracking of the mould material, interlacing minute cracks or crazesare produced which preclude shrinkage of the mould material during itssolidification. Therefore, if, by chance, an external shrinkage shouldoccurs, such shrinkage is very negligible. The rapid increase of thetemperature to which the mould material is subjected has the effect ofaccelerating the action of internal microcracking, which latter is veryminute but very uniform and produces uniform granulation of the mouldmaterial.

There are definite reasons for the necessity of firing the mould beforepouring the casting, one of them being the rapid driving off of alltraces of volatiles. If ignition or firing is not effected rapidly andalmost immediately upon the solidification of the mould material, themould material will disintegrate or at least become seriously weakened.Ignition therefore serves the dual purpose of driving off the volatilesand to provide the required granulitic structure of the mould material.

In consequence of the aforesaid micro-cracking of the mold material, thelatter attains a state of remarkable permeability which. facilitates theready escape of gases from the mould interior during casting, therebyobviating the necessity of constructing special gas vents used infoundry practice. Moreover it has been found that the mould producedaccording to the present method is highly resistant to thermal shock.

The rapid removal of the volatiles through application of intense flamesof heat immediately after setting of the mould material takes placeproduces what may be termed as freezing of the mould dimensions, that isa so-tospeak instantaneous setting of the mould to the requireddimensions. Furthermore, the peculiar structure of the mould materialwith its minute micro-cracks or crazing is the result of the rapidthermal shock to which the mould material is subjected by theapplication of intense flame or heat immediately. following the settingof the mould material.

Summarizing, the most essential feature of the present disclosure is theuse in the mould paste mixture of refractory material with an alkyl.silicate and an aqueous acid gelling agent or accelerator, and therapid, uniform and intense firing of the mould made from the aboveingredients immediately after the mould material has set by gelling,that is after the setting of the mould material is completed, and theelfectuating of such firing of the mould so that free air circulation isprovided at all surface portions of the mould, and wherein the rapidintense firing of the mould constitutes a severe thermal shock to themould, thereby effecting the peculiar internally porous structure of themould material resulting from the rapid burning at intense heat of thevolatiles escaping from the set mould, and which internally porousstructure thus produced causes the dimensional freezing of the mould,and wherein the thus produced mould is rendered what may be termedimmune to subsequent severe thermal shocks.

While specific ingredients are given in the foregoing as well asspecific steps prescribed in the production of the mould, it is readilyapparent that other materials equal in their co-action to those givenmay be readily employed, and by the same token the method steps mayundergo changes and modifications which may become necessary in theproduction of moulds for specific purposes. For the above reasons theforegoing specific disclosures are not intended to limit the scope ofthe present invention except as defined in the annexed claims.

What is claimed as new is:

1. Method of producing refractory moulds which comprises preparing aslurry comprising comminuted refractory materials and a binder of aliquid lower alkyl silicate, water and a gelling accelerator, pouringsaid slurry over a pattern, allowing the slurry to gel, immediatelyseparating the gelled mass of the slurry from the pattern, andimmediately thereafter igniting the surfaces of the gelled mass andallowing the intense flames resulting from said ignition to burn untilthe fiammables are consumed, thereby freezing the mould dimensions andobtaining a mould having a porous structure of microcracks which willnot allow cast metal to fill the crack formation but which willfacilitate the ready escape of gases during pouring.

2. Method in accordance with claim 1, wherein the alkyl silicate isethyl silicate.

3. Method of producing refractory moulds which comprises preparing aslurry comprising comminuted refractory materials and a binder of aliquid lower alkyl silicate, water and a gelling accelerator, pouringsaid slurry over a pattern, allowing the slurry to gel, immediatelyseparating the gelled mass of the slurry from the pattern, immediatelyplacing the separated gelled mass upon a supporting surface whichpermits free air circulation about all surfaces thereof, immediatelyigniting the surfaces of the gelled mass and allowing the flamesresulting from said ignition to burn until the flammables are consumed,thereby freezing the mould dimensions and obtaining a mould having aporous structure of microcracks which will not allow cast metal to fillthe crack formation but which will facilitate the ready escape of gasesduring pouring.

4. Method in accordance with claim 3 wherein alkyl silicate is ethylsilicate.

5. A mould as produced in accordance with the method of claim 4.

6. A mould as produced in accordance with the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS1,909,008 Prange May 16, 1933 2,027,932 Ray Jan. 14, 1936 2,388,299Thielemann Nov. 6, 1945 2,491,096 Feagin D66. 13, 1949 2,496,170 MannJan. 31, 1950 FOREIGN PATENTS 183,874 Austria Nov. 25, 1955 OTHERREFERENCES The Iron Age, Nov. 9, 1944, pages 52-5 8. The Iron Age, Feb.8, 1945, pages 52-57.

1. A METHOD OF PRODUCING REFRACTORY MOULDS WHICH COMPRISES PREPARING ASLURRY COMPRISING COMMINUTED REFRACTORY MATERIALS AND A BINDER OF ALIQUID LOWER ALKYL SILICATE, WATER AND A GELLING ACCELERATOR, POURINGSAID SLURRY OVER A PATTERN, ALLOWING THE SLURRY TO GEL, IMMEDIATELYSEPARATING THE GELLED MASS OF THE SLURRY FROM THE PATTERN, ANDIMMEDIATELY THEREAFTER IGNITING THE SURFACES OF THE GELLED MASS ANDALLOWING THE INTENSE FLAMES, RESULTING FROM SAID IGNITION TO BURN UNTILTHE FLAMMABLES ARE CONSUMED, THEREBY FREEZING THE MOULD DIMENSIONS ANDOBTAINING A MOULD HAVING A POROUS STRUCTURE OF MICROCRACKS WHICH WILLNOT ALLOW CAST METAL TO FILL THE CRACK FORMATION BUT WHICH WILLFACILITATE THE READY ESCAPE OF GASES DURING POURING.